A worldwide coalition of scientists launched a broad effort yesterday to understand human genetic variation, vowing to create a new type of gene map that may propel medical research forward by explaining such common ailments as heart attacks, diabetes and obesity.

The three-year, $100 million project is one of the biggest scientific undertakings since the Human Genome Project, out of which it grows. Scientists will study genes from people in Japan, China, Africa and the United States, a geographic range that they believe will reveal most, if not all, of the common human genetic variations.

The analytical work will be carried out in public and private laboratories in the United States, Britain, Japan, Canada and China.

The project could have "a profound impact on the future of medicine," said Francis S. Collins, director of the National Human Genome Research Institute in Bethesda, a prime sponsor. He said at a news conference in Washington yesterday that it was an essential next step in turning the promise of the Human Genome Project into concrete health information.

The Human Genome Project, which produced draft genetic maps two years ago amid White House fanfare, is an effort to determine the genetic makeup of what amounts to a theoretical average person. With a nearly complete reference genome in hand, scientists now need to know the ways in which people diverge from that average -- and to determine which of those patterns can be linked to illness.

Some efforts have been made in this direction already, particularly by a coalition of public laboratories and large drug companies. But the new project is designed to be more complete.

It is expected to take advantage of a key discovery about human genetic variability. Instead of being transmitted to succeeding generations at random, genes tend to be passed down in large blocks. These gene groups are known as haplotypes, and the project announced yesterday is called the International HapMap Project.

For a given haplotype, scientists expect to find only three or four common variations, even in ethnic groups separated by oceans and thousands of years of evolution. The subtle genetic differences among these variants are believed to produce subtly different chemistry in people's bodies, and the cumulative effects over a lifetime may help explain why some people suffer from heart attacks, others from cancer, and so forth.

Scientists have long hunted associations between genes and illness, with notable success in diseases caused by a single defect.

But mankind's great killers are believed to be caused, or accelerated, by patterns of a dozen or so genes working together, and scientists have had a harder time nailing down those associations, in part because they lacked a catalogue of the common variants.

"The HapMap project will create a powerful tool for linking differences in the genome to differences in health, including increased risk for common illnesses," said Huanming Yang, director of a genetic institute in Beijing and leader of China's contribution to the project.

There is potential for controversy. The HapMap project superficially resembles the Human Genome Diversity Project, an attempt more than a decade ago to use genetic sampling to determine relationships of the world's ethnic groups and, particularly, to determine how small, isolated groups might differ biologically from the mainstream.

That project was eventually scrapped after a storm of protest in poor countries that it might lead to exploitation of vulnerable populations, including patents in Western countries on the genetic variations found in remote tribes. Opponents labeled it the "vampire project."

HapMap organizers, including ethicists, said yesterday they had taken pains to avoid repeating the mistakes of the Human Genome Diversity Project. Their goals are medical, not anthropological. They will carefully solicit informed consent from genetic donors and go to great lengths to protect identities, the organizers said.

And the donors are expected to come from large groups: people of Han Chinese and Japanese ancestry, people from the Yoruba group in Nigeria, and Americans of European ancestry.

Francis S. Collins called the new project an essential next step in genetics research.